This invention relates to radio receivers, and particularly but not exclusively to such receivers forming part of the transceiver of a portable radio telephone or handset, for communication with a base station in a radio telephone system. The invention also relates to a method of operation of such radio receivers. The invention also relates, particularly but not exclusively, to handsets for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots.
Any radio system in which information is transmitted in bursts causes problems for the receiving station in maintaining synchronicity with the signals received. Many protocols provide for a solution to this problem by including a synchronisation field at the start of a burst to allow the receiving station to synchronise with the signal before signals carrying data or other information are received.
In digital systems the receiver must achieve both xe2x80x98lock recoveryxe2x80x99 or bit synchronisation with the incoming signal allowing data to be recovered, and slot, or burst, synchronisation that allows the receiver to determine which point of a burst is being received. For this purpose, a synchronisation field comprising a known string of bits is provided at the beginning of each burst so that the receiving station can correlate its timing to that of the incoming signal. The known string allows bit synchronisation to be achieved relatively quickly in good conditions ie when the signal is of a high quality. The bit synchronisation field must, however, be long enough for bit synchronisation to be achieved under all the circumstances in which the signal quality is suitable to provide information. Following the bit synchronisation string is typically a slot synchronisation string that allows the receiving station to locate a particular point in the burst so that the category or type of information carried by the subsequently received bits of that burst can be identified by virtue of their position within the burst.
Signalling protocols typically provide for error detection so that good data received can be distinguished from corrupt data. The quality of a channel for receiving signals can be monitored to determine if a new channel is necessary in order to preserve transfer of the information carried by the signal bursts from the base station to the handset or vice versa. It is desirable to monitor channel quality to allow a better quality channel to be used before too much information is lost. The quality of a received signal can be used as a trigger for changing channel.
In accordance with the present invention there is provided a radio receiver for communication in a radio system in which information is transmitted in bursts, a burst having a field for enabling the radio receiver to synchronise with an incoming signal, the radio receiver comprising processing means operable to utilise the interval for the achievement of synchronisation as a measure of signal quality.
Because a synchronisation field is typically long enough to enable synchronisation to be achieved when signal quality is less than perfect, there is some redundancy built into the signal ie it is longer than necessary for synchronisation under good conditions. This provides scope for the receiver to use information on the interval for achieving synchronisation to provide information on signal quality.
This information can be used instead of, or as well as, other signal quality information that can be obtained. The advantage of using the synchronisation field to measure signal quality is that a decision to change channels may be made before any data is lost.
When used in accordance with the present invention, the synchronisation field is particularly useful in detecting sliding collisions that may cause the loss of information at the front end of a data burst. Sliding collisions can arise when systems having overlapping spheres of influence operate under the same protocol. This is because a burst in a time slot on one system may occur at a similar time as a burst in a time slot of another system. When the two systems are unsynchronised, the timing of one system will inevitably drift relative to the other, causing the interference pattern to differ with time. A pair of bearer channels utilised on respective systems can, therefore, xe2x80x98slidexe2x80x99 in and out of interference with one another over a period of time. This type of interference is known as a xe2x80x98sliding collisionxe2x80x99.
Because there are typically bounds imposed on the accuracy of timing in respective systems, data loss as a result of a sliding collision will increase relatively slowly over a number of bursts. This allows, if desirable, a number of bursts to be used to determine if a sliding collision is occurring whilst still providing the opportunity to change channel on such a determination before a significant amount of information carried by a burst is lost.
In accordance with a second aspect of the invention there is provided a method for providing a measure of signal quality in a radio telephone system in which information is transmitted in bursts, a burst having a synchronisation field for enabling synchronisation with an incoming signal, the method comprising utilising the interval for achievement of synchronisation as a measure of signal quality.